1. Field of the Invention
The present invention relates to a magnetic recording medium. More particularly, the present invention relates to a magnetic recording medium having improved durability, abrasion resistance and surface smoothness of a magnetic layer and good cleaning effect on a magnetic head.
2. Description of the Related Art
A magnetic recording medium such as a magnetic tape, a magnetic disc and a magnetic sheet is now widely used for audio use, video recording use and computer use.
To a magnetic layer of such magnetic recording medium, non-magnetic powder such as Al.sub.2 O.sub.3, Cr.sub.2 O.sub.3, SioC or .alpha.-Fe.sub.2 O.sub.3 is added as a reinforcing material or an abrasive to increase durability and abrasion resistance of the magnetic layer or to clean foreign particles adhered to the magnetic head.
To this end, the non-magnetic powder having an average particle size of about 0.5 .mu.m is added to the magnetic layer. Recently, to increase a recording density, metal magnetic powder or fine particle iron oxide magnetic powder is used, and it is required to increase the surface smoothness and electromagnetic conversion characteristics of the magnetic recording medium. When the conventional non-magnetic powder is used, since it has a larger particle size than the magnetic powder, the electromagnetic conversion characteristics is deteriorated and the surface smoothness is worsened, and the abrasion or wear of the magnetic head is increased.
To overcome such drawbacks, it may be contemplated to use non-magnetic powder having a very small particle size of, for example, 0.4 .mu.m or less. However, the non-magnetic powder having a very small particle size does not greatly contribute to the improvement of the durability and abrasion resistance of the magnetic layer, or has small cleaning effect on the magnetic head.
To improve such defects, it was proposed to use inorganic powder having an average primary particle size of 0.35 .mu.m or less, an angle of repose of 40 degrees or larger and Mohs' hardness of 5 or larger (cf. Japanese Patent Kokai Publication No. 104427/1986), angular .alpha.-Al.sub.2 O.sub.3 0.1 to 0.3 .mu.m in average particle size and having an alpha-alumina content of at least 50 % in the crystal (cf. Japanese Patent Kokai Publication No. 250518/1987) or angular .alpha.-Al.sub.2 O.sub.3 having an average particle size of a half or less of that of the magnetic powder and less than 0.1 .mu.m (cf. Japanese Patent Kokai Publication No. 74118/1988).
Although these proposed inorganic powders can improve the cleaning effect due to their angular shape, they cannot improve the durability and abrasion resistance of the magnetic layer of the magnetic recording medium for high density recording which utilizes metal magnetic powder or fine particle iron oxide magnetic powder.
There was proposed a magnetic recording medium in which particles of inorganic powder such as Al.sub.2 O.sub.3 are coated with chromium oxide, iron oxide, zinc oxide, titanium oxide or vanadium oxide or a mixture of such oxide and other material and used in the magnetic layer to improve the surface smoothness and electromagnetic conversion characteristics (cf. Japanese Patent Kokai Publication Nos. 159236/1983 and 239616/1988). However, such coated powder has decreased abrasion effect and cannot achieve the desired magnetic head cleaning effect since the surface hardness of Al.sub.2 O.sub.3 may be hindered by the coating.